Disable Group avatar feature if WordPress show avatars setting is disabled

In WordPress Administration Discussion settings, if the option "Show Avatars" is not on, we need to disable the Group Avatar feature to avoid some notices and be consistent with the discussion option.

Fixes #5345
See #5494

git-svn-id: https://buddypress.svn.wordpress.org/trunk@8737 cdf35c40-ae34-48e0-9cc9-0c9da1808c22